There is no essential difference between pleats and purlins. Many times, it is just a convention, and it cannot be completely and strictly distinguished. But on the whole, it is more considered as formal clothes, pleats are more considered as regular clothes, and occasionally getting married is also different. From the style, Bi is a big collar, embroidered like a cloud, and the pleats are small collars. Pleats can be worn inside, but Bi can't.